The National Building Code of Canada requires Canadian homes to be ventilated mechanically. Heat Recovery Ventilator/Energy Recovery Ventilator (HRV/ERV) technology provides a mechanism to meet these ventilation requirements in an energy-efficient manner.
ERVs have the ability to transfer moisture through membranes. NRC studies showed that ERVs may have benefits over HRVs both in cold dry winters and humid summers due to this feature.
HRVs vs. ERVs in summer
A summer field study on HRV and ERV performance for a range of summer conditions was carried out using the twin houses at the NRC Canadian Centre for Housing Technology in Ottawa. NRC’s researchers compared the performance of a home equipped with an ERV to an identical side-by-side home operated with an HRV. Both units had the same exhaust pickups in bathrooms and kitchen. The ERV was found to provide better humidity control (i.e., providing a lower level of indoor relative humidity), as well as lower air conditioning electricity consumption, measured as the cumulative saving of 12% over a week.
The study also showed that there is a small potential for ERVs to introduce some bathroom moisture back into the house for a few minutes per day under certain conditions. However, the benefits of the efficiency in continuous pre-cooling and pre-dehumidifying of the incoming outdoor air and control of humidity throughout the house on a continuous basis are overriding this effect.
Managing relative humidity in winter
A winter field study investigating the impact of ventilation rates on indoor air quality and the respiratory health of asthmatic children in Québec City was carried out in over 100 homes with the Institut national de santé publique du Québec (see Construction Innovation, December 2011).
As the majority of homes in the study were found to be under-ventilated, HRVs or ERVs were subsequently installed to provide increased ventilation. In a large number of homes, the relative humidity (RH) was found to be too low in winter. Because the introduction of more cold, dry outside air further reduces RH, low-RH homes were equipped with ERVs (instead of HRVs) to increase the ventilation rate.
Both HRVs and ERVs performed equally in terms of providing better indoor air quality, characterized by a significant reduction in the concentration of a number of gaseous pollutants of indoor origin. However, ERVs were additionally effective in maintaining an acceptable indoor RH.
Summary of findings
NRC studies indicate that in certain climates, ERVs can perform better than HRVs. In cold and dry climates, ERVs are effective in preventing indoor dryness by retaining indoor humidity. In warm and humid climates, ERVs provide better humidity control and reduce the electricity consumption of air conditioning systems or dedicated dehumidifiers.
Research collaboration needed
Canadian regions vary in climates, so there is no single ideal solution for every situation. More research is needed, taking into account different regions and internal moisture loads due to occupants and their activities.
NRC welcomes partnership with clients and collaborators seeking further performance improvement of HRV/ERV technology, including performance under different climatic conditions. Experimental results will be used to develop consensus-based best practices for the effective choice of HRV/ERV systems and to develop an energy analysis tool to evaluate energy consumption and energy savings.
Contact Boualem Ouazia at firstname.lastname@example.org or 613-993-9613.